Dark Knight, how come Harvey Dent if chopped in half and replaced with a half dead person?

I just finished watching the Dark Knight. The only part I didn't get is how Harvey Dent ended up with half of a dead person attached to him. He is lying down in the hospital bead, turns around, and half of him is dead. Did they take the dead body of the Commissioner or one of the other people that the joker killed and sew it onto him? I just didn't understand that part.